The fuel assembly in the nuclear power plant generally includes five major components comprising upper nozzle, guiding tubes, grid spacers, fuel rods and lower nozzle. Wherein, a grid space is always assembled from a number of grid straps. For a 17×17 lattice grid spacer design, it has 17*17=264 grid cells. There are twenty-four guide thimble tubes and one instrument tube placed in parallel in 25 selected grid cells of the grid spacer. Thus, the grid spacer successfully separates the guide thimble tubes to provide control and spacing between the guide thimble and instrument tubes, as a result, a tube bundle can be constituted. The upper and lower ends of the tube bundle are attached with the upper nozzle and the lower nozzle, respectively, to form a skeleton structure of the fuel assembly. The fuel rods are placed in parallel in all remaining grid cells. The coolant flows through the fuel bundle from the bottom of the lower nozzle and out from the upper nozzle, moderating the neutrons and carrying away the released energy due to nuclear fission, and thus playing an important role in cooling for the fuel rods.
In order to increase the heat transfer between the coolant and the fuel rods, mixing vanes are placed at the corner of grid cell in which a fuel rod resides, to increase the cooling effects made by the coolant flow to the fuel rods. FIG. 1 is a schematic diagram of a unit grid strap cell 1 in prior art. Wherein, a mixing element is at the corner of the grid cell through a slot and some slot holes are set to increase the flow path for the coolant. Besides, adjusting the mixing vane area and angle etc. of the mixing vane are intended to improve the heat transfer efficiency. However, the structure of the existing mixing vanes has at least the following deficiencies: 1, it is difficult to direct the coolant from adjacent grid cells to provide flow mixing, so that heat transfer may not be adequate; 2, in a grid cell with having single coolant flow coolant passage, there exists ineffective vortices. The heat exchange is inadequate and insufficient; 3, during flowing coolant mixing, pressure drops are high.